Contactless distance measurements are widely used in the industry. Different technologies are deployed, dependant on the specific application needs. Technologies such as laser triangulation, confocal, fiber based, interferometer, and chromatic are common in the field of distance or displacement measurement, and are implemented by using optical methods. Each of the technologies are chosen to fit specific application requirements. For example, some of the computer-to-plate (CTP) imaging machines use the laser triangulation principle in order to focus the imaging head. One of the drawbacks of such a sensing device is the relatively high cost and form factor aspects, which impose substantial imaging head design constraints.
Another method of non-contact displacement measurement allowing small sensor dimensions is disclosed in U.S. Pat. Nos. 7,071,460 (Rush); 4,739,161 (Moriyama et al.); 5,017,772 (Hafle); and 4,801,799 (Tromborg et al.). All the disclosed patents use two or more optical fibers for measuring the distance to the media. Each of the patents disclosed are based on a predetermined media orientation in respect to the sensor. For applications where media orientation is not predetermined, for example, computer-to-plate (CTP) head calibration, such an assumption is not valid and it is impossible to accurately measure the distance to arbitrary oriented media.
FIG. 1A shows the functionality of a sensor according to U.S. Pat. No. 4,801,799, with media 130 oriented at a 90 degree angle to the sensor optical axis. FIG. 1B shows media 130 oriented at an angle other than 90 degrees. In both cases, the distance between media 130 and the outlet of the optical fiber 110 is identical.
It is apparent from FIG. 1A that the light coming from the light source 100 through fiber 110 and lens 120 is reflected back from media 130, which is oriented perpendicular with respect to the fiber light emission axis, and returns through lens 120 and fiber 140 to the light sensor circuitry 150. In this case the quantity of the reflected light energy detected by the light sensor 150 is a function of media-to-fiber outlet distance. (See FIG. 4 of U.S. Pat. No. 4,801,799.)
FIG. 1B shows the sensor functionality with a tilted orientation of media 130. In this case the light sensor 150 will receive smaller amounts of the reflected light energy or none at all. As it can be seen from FIG. 1B even a slight change in the media 130 orientation angle might lead to the deviation of light sensor 150 output signal even though the distance between the media and the fiber outlet does not change. In other words the media 130 orientation angle may significantly change the light sensor 150 output signals thus increasing the measuring errors or making measurements impossible.
In head-to-media distance measurement applications, the orientation or shape of the surface, or head to sleeve distance measurement, often varies. In those cases the fiber sensor errors caused by media orientation or shape variances becomes a substantial disadvantage of the measurement sensor. Additionally, imaging head alignment deviations or sleeve or drum eccentricity changes are also affect media-to-sensor distance measurements.